The release of glutamate at synapses at many sites in mammalian forebrain stimulates two classes of postsynaptic ionotropic glutamate receptors. These classes are usually referred to as AMPA and N-methyl-D-aspartic acid (NMDA) receptors. AMPA receptors mediate a voltage independent fast excitatory post-synaptic current (the fast EPSC), whereas NMDA receptors generate a voltage-dependent, slow excitatory current. Studies carried out in slices of hippocampus or cortex, indicate that the AMPA receptor mediated fast EPSC is generally the dominant component by far at most glutamatergic synapses, and activation of AMPA receptors is usually a prerequisite for NMDA receptors activation.
AMPA receptors are expressed throughout the central nervous system. These receptors are found in high concentrations in the superficial layers of neocortex, in each of the major synaptic zones of hippocampus, and in the striatal complex, as reported by Monaghan et al., in Brain Research 324:160-164 (1984). Studies in animals and humans indicate that these structures organize complex perceptual-motor processes and provide the substrates for higher-order behaviors. Thus, AMPA receptors mediate transmission in those brain networks responsible for a host of cognitive activities. In addition, AMPA receptors are expressed in brain regions that regulate the inspiratory drive responsible for control of breathing (Paarmann et al, Journal of Neurochemistry, 74: 1335-1345 (2000).
For the reasons set forth above, drugs that modulate and thereby enhance the functioning of AMPA receptors could have significant benefits for intellectual performance as well as reversal of respiratory depression induced by pharmacological agents such as opioids and opiates, or other means. Such drugs should also facilitate memory encoding. Experimental studies, such as those reported by Arai and Lynch, Brain Research 598:173-184 (1992), indicate that increasing the size of AMPA receptor-mediated synaptic response(s) enhances the induction of long-term potentiation (LTP). LTP is a stable increase in the strength of synaptic contacts that follows repetitive physiological activity of a type known to occur in the brain during learning.
Compounds that enhance the functioning of the AMPA subtype of glutamate receptors facilitate the induction of LTP and the acquisition of learned tasks as measured by a number of paradigms. See, for example, Granger et al., Synapse 15:326-329 (1993); Staubli et al., PNAS 91:777-781 (1994); Arai et al., Brain Res. 638:343-346 (1994); Staubli et al., PNAS 91:11158-11162 (1994); Shors et al., Neurosci. Let. 186:153-156 (1995); Larson et al., J. Neurosci. 15:8023-8030 (1995); Granger et al., Synapse 22:332-337 (1996); Arai et al., JPET 278:627-638 (1996); Lynch et al., Internat. Clin. Psychopharm. 11:13-19 (1996); Lynch et al., Exp. Neurology 145:89-92 (1997); Ingvar et al., Exp. Neurology 146:553-559 (1997); Hampson, et al., J. Neurosci. 18:2748-2763 (1998); Porrino et al., PLoS Biol 3(9): 1-14 (2006) and Lynch and Rogers, U.S. Pat. No. 5,747,492. There is a considerable body of evidence showing that LTP is the substrate of memory. For example, compounds that block LTP interfere with memory formation in animals, and certain drugs that disrupt learning in humans antagonize the stabilization of LTP, as reported by del Cerro and Lynch, Neuroscience 49: 1-6 (1992). Learning a simple task induces LTP in hippocampus that occludes LTP generated by high frequency stimulation (Whitlock et al., Science 313:1093-1097 (2006)) and a mechanism that maintains LTP sustains spatial memory (Pastalkova, et al., Science 313:1141-1144 (2006)). Of significant importance to the field of learning is the finding that in vivo treatments with a positive AMPA-type glutamate receptor modulator restores stabilization of basal dendritic LTP in middle-aged animals (Rex, et al., J. Neurophysiol. 96:677-685 (2006)).
Drugs that enhance the functioning of the AMPA receptor can effectively reverse opioid- and barbiturate-induced respiratory depression without reversing the analgesic response (Ren et al, American Journal of Respiratory and Critical Care Medicine, 174: 1384-1391 (2006). Therefore these drugs may be useful in preventing or reversing opioid-induced respiratory depression and for alleviating other forms of respiratory depression including sedative use and sleep apnea. Excitatory synaptic transmission provides a major pathway by which neurotrophic factors are increased within specific brain regions. As such, potentiation of AMPA receptor function by modulators has been found to increase levels of neurotrophins, particularly brain derived neurotrophic factor, or BDNF. See, for example, Lauterborn, et al., J. Neurosci. 20:8-21 (2000); Gall, et al., U.S. Pat. No. 6,030,968; Lauterborn, et al., JPET 307:297-305 (2003); and Mackowiak, et al., Neuropharmacology 43:1-10 (2002). Other studies have linked BDNF levels to a number of neurological disorders, such as Parkinson's disease, Attention Deficit Hyperactivity Disorder (ADHD), autism, Fragile-X Syndrome, and Rett Syndrome (RTT). See, for example, O'Neill, et al., Eur. J. Pharmacol. 486:163-174 (2004); Kent, et al., Mol. Psychiatry. 10:939-943 (2005); Riikonen, et al., J. Child Neurol. 18:693-697 (2003) and Chang, et al., Neuron 49:341-348 (2006). Thus, AMPA receptor potentiators may be useful for the treatment of these, as well as other, neurological diseases that are the result of a glutamatergic imbalance or a deficit in neurotrophic factors.
A prototype for a compound that selectively facilitates the AMPA receptor has been described by Ito et al., J. Physiol. 424:533-543 (1990). These authors found that the nootropic drug aniracetam (N-anisoyl-2-pyrrolidinone) increases currents mediated by brain AMPA receptors expressed in Xenopus oocytes without affecting responses by γ-aminobutyric acid (GABA), kainic acid (KA), or NMDA receptors. Infusion of aniracetam into slices of hippocampus was also shown to substantially increase the size of fast synaptic potentials without altering resting membrane properties. It has since been confirmed that aniracetam enhances synaptic responses at several sites in hippocampus, and that it has no effect on NMDA-receptor mediated potentials (Staubli et al., Psychobiology 18:377-381 (1990) and Xiao et al., Hippocampus 1:373-380 (1991)).
Aniracetam has been found to have an extremely rapid onset and washout, and can be applied repeatedly with no apparent lasting effects, which are desirable features for behaviorally-relevant drugs. Aniracetam does present several disadvantages, however. The peripheral administration of aniracetam is not likely to influence brain receptors. The drug works only at high concentrations (approx. 1000 μM), and about 80% of the drug is converted to anisoyl-GABA following peripheral administration in humans (Guenzi and Zanetti, J. Chromatogr. 530:397-406 (1990)). The metabolite, anisoyl-GABA, has been found to have less activity than aniracetam. In addition to these issues, aniracetam has putative effects on a plethora of other neurotransmitter and enzymatic targets in the brain, which makes uncertain the mechanism of any claimed therapeutic drug effect. See, for example, Himori, et al., Pharmacology Biochemistry and Behavior 47:219-225 (1994); Pizzi et al., J. Neurochem. 61:683-689 (1993); Nakamura and Shirane, Eur. J. Pharmacol. 380: 81-89 (1999); Spignoli and Pepeu, Pharmacol. Biochem. Behav. 27:491-495 (1987); Hall and Von Voigtlander, Neuropharmacology 26:1573-1579 (1987); and Yoshimoto et al., J. Pharmacobiodyn. 10:730-735 (1987).
A class of AMPA receptor-enhancing compounds that does not display the low potency and inherent instability characteristic of aniracetam has been described (Lynch and Rogers, U.S. Pat. No. 5,747,492). These compounds, termed “Ampakines”R, can be substituted benzamides which include, for example, 6-(piperidin-1-ylcarbonyl)quinoxaline (CX516; AmpalexR). Typically, they are chemically more stable than aniracetam and show improved bio-availability. CX516 is active in animal tests used to detect efficacious drugs for the treatment of memory disorders, schizophrenia, and depression. In three separate clinical trials, CX516 showed evidence for efficacy in improving various forms of human memory (Lynch et al., Internat. Clin. Psychopharm. 11:13-19 (1996); Lynch et al., Exp. Neurology 145:89-92 (1997); Ingvar et al., Exp. Neurology 146:553-559 (1997)).
Another class of Ampakines, benzoxazines, has been discovered to have very high activity in in vitro and in vivo models for assessing the probability of producing cognition enhancement (Rogers and Lynch; U.S. Pat. No. 5,736,543). The substituted benzoxazines are rigid benzamide analogues with different receptor modulating properties from the flexible benzamide, CX516.
Certain substituted 2.1.3 benzoxadiazole compounds have been found to be significantly and surprisingly more potent in animal models of attention deficit hyperactivity disorder (ADHD), schizophrenia and cognition than previously disclosed compounds in US 2002/0055508 and US 2002/0099050. This new class of N,N-disubstituted amides (I) display significant activity for enhancing AMPA mediated glutamateric synaptic responses.
